The present invention relates to a method for producing low-chloride plaster products from impure products or residues, which contain calcium and/or calcium compounds, such as sulfites, chlorides, sulfates, hydroxides, carbonates and/or oxides of calcium or mixtures thereof.
Calcium-containing residues are formed, for example, during the removal of sulfur dioxide (SO.sub.2) and other environmentally harmful components from flue-gases or waste gases of coal-fired plants, combustion plants and remelting plants. Methods for the thermal conversion of such residues into plaster products by oxidation and calcining are known. However, residues with high calcium chloride content cannot be processed into high-grade, low-chloride plaster products using known methods. High-grade, low-chloride plasters are useful, for example, in the building industry in the manufacture of calcium sulfite anhydrate for building cement and retarders.
Some calcium-containing residues, such as the spray absorption residues from dry flue-gas purification plants, have CaCl.sub.2 contents of up to 10% by weight. Spray-absorption residues originate from dry or quasi-dry desulfurization installations, which are operated with lime as an absorbent. A method to produce low-chloride anhydrous plaster from such residues is disclosed in U.S. Pat. No. 4,495,162 (European Patent 0,074,772.) The method disclosed in the '162 patent includes the oxidation of a CaSO.sub.3 component of the residue to CaSO.sub.4 (calcium anhydrite). Then, in a separate step, chlorine is removed by exposing the residue to steam at temperatures between 600.degree. and 950.degree. C. To achieve a low chloride content in the product manufactured by this method, large quantities of fly ash or materials containing SiO.sub.2 and Al.sub.2 O.sub.3 are required. Addition of these materials functions to bind the calcium chloride (CaCl.sub.2) components as calcium silicate or calcium aluminate. This is said to facilitate the reaction of the chlorine liberated from the residue and water (steam) to form gaseous HCl.
Despite the availability of this process, plaster products having chloride contents of less than 0.5% by weight are produced only at temperatures well in excess of 600.degree. C. A further disadvantage of this method is the resulting high proportion of silicate and aluminate in the end product.
It is an object of the present invention to provide an effective method for processing calcium-containing impure products or residues to form low-chloride plaster having a calcium chloride content below 0.5% by weight.
Another object of the present invention is to provide a method for making low-chloride plasters from calcium containing residues that contain up to 10% by weight of CaCl.sub.2 at temperatures between 500.degree. C. and 600.degree. C.
A further object of the present invention is to provide a process for making high grade plaster products containing less than 0.5% by weight of chlorine from calcium containing waste materials containing up to 10% by weight of CaCl.sub.2, that is not dependent on the quantity of fly ash, SiO.sub.2 or Al.sub.2 O.sub.3 that is present in the starting material.
A still further object of the invention is to produce low-chloride plaster in either anhydrous form or in a partially hydrated form (beta-hemihydrate form).
These and other objects of the present invention will be apparent to those of ordinary skill in the art in light of the following description, drawings and claims.